1. Field
The present invention relates generally to data communication, and more specifically to techniques for allocating downlink resources in a multiple-input multiple-output (MIMO) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on, for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for transmission of multiple independent data streams. In one common MIMO system implementation, the data streams are transmitted to a single terminal at any given time. However, a multiple access communication system having a base station with multiple antennas may also concurrently communicate with a number of terminals. In this case, the base station employs a number of antennas and each terminal employs NR antennas to receive one or more of the multiple data streams.
The connection between a multiple-antenna base station and a single multiple-antenna terminal is called a MIMO channel. A MIMO channel formed by these NT transmit and NR receive antennas may be decomposed into NC independent channels, with NCxe2x89xa6min {NT, NR}. Each of the NC independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities of these subchannels created by the multiple transmit and receive antennas are utilized.
Each MIMO channel between the base station and a terminal typically experiences different link characteristics and is associated with different transmission capability, so the spatial subchannels available to each terminal have different effective capacities. Efficient use of the available downlink resources (and higher throughput) may be achieved if the NC available spatial subchannels are effectively allocated such that data is transmitted on these subchannels to a xe2x80x9cproperxe2x80x9d set of terminals in the MIMO system.
There is therefore a need in the art for techniques to allocate downlink resources in a MIMO system to provide improved system performance.
Aspects of the invention provide techniques to increase the downlink performance of a wireless communication system. In an aspect, data may be transmitted from a base station to one or more terminals using one of a number of different operating modes. In a MIMO mode, all available downlink data streams are allocated to a single terminal that employs multiple antennas (i.e., a MIMO terminal). In an N-SIMO mode, a single data stream is allocated to each of a number of distinct terminals, with each terminal employing multiple antennas (i.e., SIMO terminals). And in a mixed-mode, the downlink resources may be allocated to a combination of SIMO and MIMO terminals, with both types of terminals being simultaneously supported. By transmitting data simultaneously to multiple SIMO terminals, one or more MIMO terminals, or a combination thereof, the transmission capacity of the system is increased.
In another aspect, scheduling schemes are provided to schedule data transmissions to active terminals. A scheduler selects the best operating mode to use based on various factors such as, for example, the services being requested by the terminals. In addition, the scheduler can perform an additional level of optimization by selecting a particular set of terminals for simultaneous data transmission and assigning the available transmit antennas to the selected terminals such that high system performance and other requirements are achieved. Several scheduling schemes and antenna assignment schemes are provided and described below.
A specific embodiment of the invention provides a method for scheduling downlink data transmission to a number of terminals in a wireless communication system. In accordance with the method, one or more sets of terminals are formed for possible data transmission, with each set including a unique combination of one or more terminals and corresponding to a hypothesis to be evaluated. One or more sub-hypotheses may further be formed for each hypothesis, with each sub-hypothesis corresponding to specific assignments of a number of transmit antennas to the one or more terminals in the hypothesis. The performance of each sub-hypothesis is then evaluated, and one of the evaluated sub-hypotheses is selected based on their performance. The terminal(s) in the selected sub-hypothesis are then scheduled for data transmission, and data is thereafter transmitted to each scheduled terminal from one or more transmit antennas assigned to the terminal.
Each transmit antenna may be used to transmit an independent data stream. To achieve high performance, each data stream may be coded and modulated based on a scheme selected, for example, based on a signal-to-noise-plus-interference (SNR) estimate for the antenna used to transmit the data stream.
Terminals desiring data transmission (i.e., xe2x80x9cactivexe2x80x9d terminals) may be prioritized based on various metrics and factors. The priority of the active terminals may then be used to select which terminal(s) to be considered for scheduling and/or to assign the available transmit antennas to the selected terminals.
The invention further provides methods, systems, and apparatus that implement various aspects, embodiments, and features of the invention, as described in further detail below.